A 4-pole field DC miniature motor is used in a field which requires large output power, such as a field of motor-driven tools. FIG. 16 is a development view showing the principle of conventional rotor windings for a 4-pole field DC miniature motor (refer to Patent Document 1). The principle of conventional winding will be described for the case of six rotor poles and six commutator segments, taking an example of so-called multipolar winding in which winding is wound about three rotor poles. The illustrated winding is continuously performed by use of two electric wires. The solid line shows a first winding which starts from a commutator segment a, and the broken line shows a second winding which starts from a commutator segment a′.
In FIG. 16, commutator segments a and a′, b and b′, and c and c′ are respectively located radially opposite each other on a commutator. The first winding indicated by the solid line starts winding from the commutator segment a; is wound about rotor poles 3, 2, and 1 by predetermined turns; is connected (hooked) to the next adjacent commutator segment b; and is then connected to the commutator segment b′ located radially opposite the commutator segment b. A portion of the first winding which connects the commutator segments b and b′ is indicated as a short circuit wire. Then, the first winding continues from the commutator segment b′; is similarly wound about rotor poles; is connected to adjacent commutator segment c′; and is then connected to the commutator segment c located radially opposite the commutator segment c′. Then, the first winding continues from the commutator segment c and is finally connected, via the commutator segment a′, to the commutator segment a, from which winding has started. Similarly, the second winding indicated by the broken line starts from the commutator segment a′ and ends at the commutator segment a′.
FIG. 17 is a development view showing rotor windings of an actually manufactured conventional 4-pole field DC miniature motor. Conventional winding will be described for the case of 10 rotor poles and 10 commutator segments, taking an example of so-called multipolar winding in which winding is wound about three rotor poles. In FIG. 17, for clear representation, the rotor poles are shown in two vertically separated rows; however, the same number indicates the same rotor pole. The illustrated winding is continuously performed by use of two electric wires. The first winding starting from the commutator segment a is shown under a row of commutator segments, and the second winding starting from commutator a′ is shown above the row of commutator segments. In FIG. 17, S denotes a winding start point, and E denotes a winding end point. The method of winding is as shown in FIG. 17 and is basically similar to that shown in FIG. 16. Thus, description of the winding method is omitted.
A 4-pole field DC miniature motor as shown in FIG. 16 or 17 actually requires short circuit wires for connecting radially opposed commutator segments when a pair of brushes is used or when, although two pairs of brushes are used, accuracy of motor rotation is required to be improved further. The illustrated wiring method enables winding about rotor poles and connection of short circuit wires to be continuously performed by use of two electric wires.
However, such a winding method requires hooking of three electric wires at the winding start and end commutator segments. For example, in FIG. 17, electric wires gather at the commutator segment a as follows: in addition to the first hooking of a winding start wire and the second hooking of a winding end wire, an electric wire from a commutator segment e is wound about rotor poles, is third hooked to the commutator segment a, and is then connected to the commutator segment a′ located radially opposite the commutator segment a. The hooked electric wires are spot-welded to the respective commutator segments for electrical connection and mechanical fixation. In the case where rotor windings use a thick wire for implementing a motor having large output power, difficulty is encountered in spot welding.
FIG. 18 is a series of views for explaining connection of electric wires to a commutator segment. As shown in (a) of FIG. 18(A), an end portion of a commutator segment located on a cylindrical commutator core fixed on a shaft is bent, thereby forming an electric wire connection portion. Next, as shown in (b), electric wires are hooked in a clearance formed between the electric wire connection portion and the commutator segment. Next, as shown in (c), a spot welding electrode is pressed against the electric wire connection portion, followed by spot welding. However, as shown in (d), when three thick electric wires are hooked, difficulty is encountered in spot welding. If the electric wire connection portion is elongated (a groove in which electric wires are hooked is deepened) for improvement of spot weldability, the entire motor length increases. Thus, because of limitation on motor dimensions, such a dimensional increase is disabled.
According to the winding method shown in FIG. 16 or 17, in order to enable winding about rotor poles and connection of short circuit wires through continuous winding operation, radially opposed commutator segments are connected twice; i.e., radially opposed commutator segments are connected through two short circuit wires. Thus, in a region between the commutator and the rotor poles, the short circuit wires gather toward the commutator. Particularly, when a thick electric wire is used, the electric wires gather bulkily in the vicinity of the commutator segments. Also, winding appearance becomes nonuniform, resulting in a difference in winding resistance among windings. Therefore, performance (current) becomes unstable.
FIGS. 19(A) and 19(B) are a pair of conceptual views for explaining the nonuniformity of winding appearance, showing windings as viewed from a rotor thrust direction. FIG. 19(A) shows a case where, by use of two electric wires, six windings are continuously wound through simultaneous winding of every two windings. FIG. 19(B) shows a case where, by use of one electric wire, six windings are continuously wound. In either case, multipolar winding is employed, and numbers assigned to windings indicated by respective ellipses in FIGS. 19(A) and 19(B) indicate the sequence of winding. A lateral projection of a winding 1, which is wound first, is small. Since a subsequent winding is wound on the preceding winding, a lateral projection of the subsequent winding increases (winding appearance becomes nonuniform). The nonuniformity of winding appearance is accelerated through bulky gathering of short circuit wires in the vicinity of commutator segments and through use of a thick electric wire. Since the electric resistance of each winding connected between two commutator segments depends on the entire length of winding, the electric resistance of each winding sequentially increases toward the last wound winding. In a 4-pole field DC motor, current flows to two radially opposed windings at the same timing, thereby compositely generating magnetic fields of the same direction. Particularly, when, as shown in FIG. 19(A), two windings of small resistance and two windings of large resistance are disposed respectively in a radially opposed manner, current which flows during the course of one revolution of rotor fluctuates greatly.
FIG. 20 is a view showing fluctuations of current waveform during the course of one revolution of the rotor. In FIG. 20, “prior art” shows fluctuations in measured current waveform of a miniature motor having windings wound as shown in FIG. 17. In contrast to a miniature motor of the present invention (first embodiment to be described later with reference to FIGS. 6 and 7), the prior art miniature motor has exhibited, in measurement, great fluctuations in current which flows during the course of one revolution of the rotor.
In the winding method shown in FIG. 16 or 17, the opposite ends of each winding coil wound about rotor poles are connected to respective commutator segments, and radially opposed commutator segments are connected through a short circuit wire, whereby two radially opposed windings are connected in parallel to a DC power supply. However, the series connection of windings is also known. FIG. 21 is a view for explaining an example of windings in general series connection. As shown in FIG. 21, winding starts from the commutator segment a; winding 1 is wound about the rotor poles 3, 2, and 1 by predetermined turns; continuously in series with the winding 1, a winding 2 is wound about rotor poles 8, 7, and 6; and then the winding 2 is connected to the next adjacent commutator segment b. Then, the commutator segment b is connected, through a short circuit wire, to the commutator segment b′ located radially opposite the commutator segment b. Subsequently, winding continues in a similar manner. In such series connection of windings, two windings are connected in series between adjacent commutator segments. Thus, radially opposed commutator segments are connected to each other only through one short circuit wire, so that the above-mentioned problem of hooking of three electric wires does not arise. However, the series connection of windings is inferior in performance to the parallel connection of windings. By means of halving the number of winding turns and doubling the area (thickness) of an electric wire, the series connection of windings can exhibit performance similar to that of the parallel connection of windings. However, winding a thick electric wire for a miniature motor imposes a strain on manufacture. Also, winding of a thick wire increases the degree of the nonuniformity of winding appearance; thus, a difference in resistance arises among rotor windings.    Patent Document 1: Japanese Utility Model Publication (kokoku) No. H6-2463